1. Field of the Invention
The present invention relates to a radio communication system and a communication method thereof, and more particularly to a radio communication system that conducts handovers between radio base stations at a short time and a communication method thereof.
2. Description of the Prior Art
In a mobile communication terminal that conducts radio communication while moving by means of a wireless LAN (Local Area Network) or the like, an apparatus that has a plurality of radio communication parts is known. Each radio communication part carries out the process to connect itself to the different base station to reduce the period of time in which communication cannot carried out. The period of time is generated when a mobile communication terminal moves from a base station to another base station (handover).
FIG. 12 is a schematic diagram of one example of a conventional communication system, and FIG. 13 is a schematic diagram of one example of a mobile communication terminal in the conventional communication system.
Referring to FIG. 12, the example of a conventional communication system comprises a fixed network 101, base stations 102 and 103 that are connected by wire with the fixed network 101, and a mobile communication terminal 104 which is capable of a wireless connection with the base stations 102 and 103.
Further, the mobile communication terminal 104 comprises an information processing part 111 and radio communication parts 112 and 113.
The information processing part 111 controls a connection with the base station 102 through the radio communication part 112, and controls a connection with the base station 103 through the radio communication part 113.
Referring to FIG. 13, the information processing part 111 in the mobile communication terminal 104 comprises radio communication control parts 121 and 122 and a handover control part 123.
The handover control part 123 controls the radio communication part 112 through the radio communication control part 121, and controls the radio communication part 113 through the radio communication control part 122.
The radio communication control parts 121 and 122 are software that control the radio communication parts 112 and 113, and they provide an interface for other applications in the system to carry out status setting or status acquisition of the radio communication parts 112 and 113 or to carry out transmission and reception of data using the radio communication parts 112 and 113.
The handover control part 123 is software for conducting a fast handover between the base stations 102 and 103, and it provides a virtual communication interface for other applications in the system. The handover control part 123 continuously monitors and controls the status of the two radio communication parts 112 and 113 using the radio communication control parts 121 and 122, and selects the radio communication part with the best communication quality from the radio communication parts 112 and 113, for communication with the base station 102 or 103.
When the handover control part 123 receives a request for transmission or reception of data from an application, it processes the request using the radio communication part that was selected for communication.
The handover control part 123 controls the radio communication part other than the radio communication part that was selected for communication to search the neighboring base stations and to connect to a base station that is a candidate as a handover destination in advance.
For example, when the radio communication part 112 that is connected to the base station 102 is selected for communication, the handover control part 123 controls the radio communication part 113 to search for a base station, and then connects the radio communication part 113 to the base station 103 that was found as a result of the search.
When the connection status of the radio communication part 113 becomes more favorable than that of the radio communication part 112, the handover control part 123 switches to the radio communication part 113 as the radio communication part used for communication, and controls the radio communication part 112 to search for a base station and so forth, thereby conducts a handover from the base station 102 to the base station 103.
Thus, by providing the mobile communication terminal 104 with a plurality of radio communication parts and carrying out appropriate control, the time required for a handover can be reduced to an extremely short time.
On the contrary, when only one radio communication part is provided, communication is not possible while searching for a base station or during processing to connect to a base station, and therefore a time in which communication is not possible arises at the time of a handover.
As another example of this type of prior art, technology has been disclosed in which, when a radio mobile terminal is selecting a base station for communication, it receives the respective control signals of neighboring base stations and determines the base station with the largest electric field intensity among base stations found within a predetermined time or determines a base station with an electric field intensity that exceeds a predetermined favorable electric field intensity. Once a base station for which the electric field intensity exceeds a predetermined favorable intensity is found, the search is immediately terminated, even if the predetermined time has not elapsed, and that base station is selected as the base station for communication (see Patent Document 1).
As a further example of this type of prior art, technology has been disclosed in which a mobile station 16 negotiates with a base station 14 in a cell 13 using a radio set 18 before the reception level of a radio set 17 decreases, to thereby acquire a channel of the base station 14 in advance. Then, when the reception level of the radio set 17 has decreased, a switch 19 is used to perform switching from a channel of a base station 12 to the channel of the base station 14 (see Patent Document 2).    [Patent Document 1] Japanese Patent Laid-Open No. 11-18133 (paragraphs 0008, 0018-0021 and FIG. 4)    [Patent Document 2] Japanese Patent Laid-Open No. 11-27720 (paragraph 0020 and FIG. 2)
Although it is necessary to mount a plurality of radio communication parts such as wireless LAN cards on a computer board for a mobile communication terminal having the above kind of functions, since the kinds of computer boards having an interface that enables these functions are few, problems exist in that there are few boards that satisfy required specifications and that downsizing is difficult.
Further, even when it is possible to mount a plurality of radio communication parts on a computer board, the radio communication parts are disposed extremely close to each other resulting in the problem that communication quality deteriorates due to the effects of mutual interference of the radio waves.
In the aforementioned Patent Documents 1 and 2, nothing is disclosed regarding the characteristic configuration of the present invention in which each radio communication part is disposed in a different terminal.